Fix segmentation fault on object free when using extension

[eric-hemasystems]

If a XSL file activates an extension. For example:

<?xml version='1.0' encoding='UTF-8'?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:func="http://exslt.org/functions" extension-element-prefixes="func">
</xsl:stylesheet>

Then we load two instances of the library both referencing that XSL file:

require 'xml/xslt'

xslt1 = XML::XSLT.new()
xslt1.xsl = 'xslt.xsl'

xslt2 = XML::XSLT.new()
xslt2.xsl = 'xslt.xsl'

Then we get a segmentation fault. I believe this is because of the:

xsltCleanupGlobals();
xmlCleanupParser();
xmlMemoryDump();

that is run whenever a XML::XSLT object is garbage collected. While the XSLT tutorial does suggest these methods, if you look at the documentation to xmlCleanupParser you will see it is clear this should only be done when the library is no longer needed:

Since our other instance may still be using the library it now gets an error when it tries to cleanup it's stylesheet that is using an extension.

This commit removes that freeing of memory as we don't really have any idea when the calling application might be done with the library. This is also per the recommendation in the documentation where it says:

It's sometimes very hard to guess if libxml2 is in use in the
application, some libraries or plugins may use it without notice. In
case of doubt abstain from calling this function...